a. Field of the Invention
This invention relates to control of an aircraft, and more specifically to control of an aircraft by deflecting the wings through controlled movements of wing control surfaces.
b. Description of the Prior Art
The control of high speed aircraft has been a major problem for aircraft designers. Aircraft with highly aft swept wings have poor control effectiveness, especially roll control, in the high dynamic pressure region. The difficulty of control of high speed aft swept wing aircraft in roll results from aeroelastic twist caused by trailing edge control surface deflections. Normally, wing control surfaces when differentially deflected create differential lift on the wing panels which results in a rolling moment. However, as a wing is flexible, the increase in differential lift caused by the trailing edge control surface deflections will also cause each wing to twist in a direction to reduce the differential lift. At high dynamic pressure flight conditions, the twisting due to aeroelastic effects is large. As a result, roll control surfaces must be deflected to a large degree to obtain the desired roll, thereby increasing actuation requirements which adds to aircraft weight and power system requirements. At a certain point, the effect can result in roll reversal or what is commonly known as aileron reversal. (The aircraft will actually roll in the opposite direction from the pilot's commands.) To maintain roll control effectiveness, the wings are traditionally stiffened and a rolling tail utilized. The addition of a rolling tail and increasing stiffness of the wings results in a heavier aircraft, reduced aerodynamic performance, and increased observables.
In U.S. Pat. No. 821, 393 to O. and W. Wright issued May 2, 1906, there is disclosed a maze of ropes and pulleys to twist the bi-plane wings to provide lateral stability. The wings being made from wood and covered with cloth were sufficiently flexible to allow for the desired twist. The pilot was positioned in a movable cradle to which the ropes were attached. Thus, the pilot by his lateral movement could cause rope movement and resulting wing twist. The Wright Brothers' object was to keep the wings level and control the lateral upset initiated by wind gusts. This practice has been made obselete by the current practice of aircraft control forces effected by control surface deflections with wing flexure minimized.
Another problem with high speed aircraft, is the necessity for making design compromises in the wings for effectiveness over the flight envelope. Aerodynamic requirements for different flight conditions vary. For example, optimal wing configuration for transonic maneuver and supersonic cruise are considerably different. Since current aircraft wings must normally be stiffened to provide roll control effectiveness, there is little that can be done to resolve this problem by variable aeroelastic twist. Mechanical devices to vary leading and trailing edge camber have been used in this regard, but only produce an inefficient compromise since the balance of the wing remains unaffected and aircraft weight, cost, and complexity are increased.